Abstarct: Reducing the dissipated electric energy is one of the most important issues in the distribution electricity network. The electric companies try to decrease the dissipated electric energy to reduce the cost and increase the profit of generating the electric energy. Recently, the use of the distributed generation (DG) has been increased. One of the advantages of utilizing DG is reducing the dissipated electric energy. The DGs can reduce the disputed electric energy significantly, if their size and location are chosen appropriately. In this thesis, the optimal location and size of the DGs obtained considering the loading pattern of the network in order to Reduce losses. The objective function of the proposed is optimized using the method of particle swarm optimization (PSO). In the proposed method, the arranged load curve is obtained using the load curve of the network. Then, the arranged curve is divided into three parts: light, normal and peak load. For each part, the average power and time index is calculated and then used in the objective function of the proposed method. To make realistic the case, the load growth is considered in the objective function. The proposed method is then applied to a part of real distribution network and load and number, size of and location of DGs are obtained using PSO and considering the load growth. The loading pattern of buses at the load points is assumed to be similar to the loading pattern of the network. Simulation results show the proposed method baxsed on weighted loading pattern has less dissipated electric energy compared to using other loading pattern, I.e. light, normal or peak loading pattern.